At least one hard disk drive (HDD) is used in almost all computer system operations. In fact, most computing systems are not operational without some type of HDD to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the HDD is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic HDD model includes a storage disk or hard disk that spins at a designed rotational speed. An actuator arm with a suspended slider is utilized to reach out over the disk. The slider is coupled with a suspension that supports both the body of the slider and a head assembly that has a magnetic read/write transducer or head or heads for reading/writing information to or from a location on the disk. The complete head assembly, e.g., the suspension, slider, and head, is called a head gimbal assembly (HGA).
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. There are tracks at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Over the years, the disk and the head have undergone great reductions in their size. Much of the refinement has been driven by consumer demand for smaller and more portable HDDs such as those used in personal digital assistants (PDAs), Moving Picture Experts Group audio layer 3 (MP3) players, and the like. For example, the original HDD had a disk diameter of 24 inches. Modem HDDs are much smaller and include disk diameters of less than 2.5 inches. Advances in magnetic recording are also primary reasons for the reduction in size.
A second refinement to the HDD is the increased efficiency and reduced size of the spindle motor spinning the disk. That is, as technology has reduced motor size and power draw for small motors, the mechanical portion of the HDD can be reduced and additional revolutions per minute (RPMs) can be achieved. For example, it is not uncommon for a HDD to reach speeds of 15,000 RPMs. This second refinement provides weight and size reductions to the HDD, it also provides a faster read and write rate for the disk thereby providing increased speed for accessing data. The increase in data acquisition speed due to the increased RPMs of the HDD and the more efficient read/write head portion provide modem computers with hard disk speed and storage capabilities that are continually increasing.
However, the higher RPMs of the disk have resulted in problems with respect to the interaction of the air with components of the HDD. For example, although the HDD is closed off from the outside, it has an amount of air within its packaging. As the disk spins and the RPMs increase, the air within the HDD package will also begin to rotate and will eventually approach the speed at which the disk is rotating especially near the spindle hub and disk surfaces. This is due to the friction between the disk and the air. In general, Reynolds numbers are used to represent the flow characteristics. For example, in one case the Reynolds number may be based on the tip speed of the disk. That is, the linear velocity at the outer diameter of the disk.
Only when the Reynolds number is sufficiently small (e.g., an enclosure with reduced air density), the air may stay in steady flow with the boundary layer of air remaining smooth with respect to the rotating disk. However, any obstructions to the flow will result in turbulence.
As is well known from fluid dynamics, the characteristics of turbulent airflow can include buffeting, harmonic vibration, and the like. Each of these characteristics will result in problematic motion for the arm and head portion and/or the rotating disk. The problematic motion will result in excessive track miss-registration. This is even more significant as the tolerances are further reduced. In addition to track miss, turbulent airflow can induce vibrations that can damage components of the hard disk drive over time.